Silver halide emulsions are usually prepared by precipitating silver halide (silver bromide, silver iodide, silver chloride or mixture thereof) in the presence of a hydrophilic colloid (normally gelatin).
Afterwards, the silver halide emulsions are subjected to a sensitization process for increasing their sensitivity to light. There are two primary types of sensitization processes: spectral sensitization and chemical sensitization.
Spectral sensitization comprises the addition of spectral sensitizing dyes which are adsorbed on the silver halide grain surface to make the emulsion sensitive to the imaging radiation or emitted radiation (i.e., visible or infrared radiation).
Chemical sensitization involves the addition of various chemical substances to obtain a prescribed value of sensitivity and contrast. Typical methods for chemical sensitizing a silver halide photographic emulsion include sulfur sensitization, noble metal sensitization, and reduction sensitization. It is also common in the art to have combination methods, such as sulfur-noble metal sensitization, reduction-noble metal sensitization, and the like.
A number of patents and patent applications, as well as literature references disclose specific methods to improve chemical sensitization. See i.e., Research Disclosure, September 1994, Item 36544, Paragraph IV, pp. 510-511, which gives a wide array of references for each of the above-mentioned methods.
Additionally, a wide range of metals have been used as doping agents during emulsion making of silver halide emulsions to modify grain structure and properties. A general review of the use of these doping agents can be found in Research Disclosure, September 1994, Item 36544, Paragraph II.D.3, pp. 504-505.
Some recent patents and patent applications teach the use of some metals during chemical sensitization alone or in combination with the above-mentioned conventional method.
For example, EP 467,106 discloses a silver halide photographic element chemically sensitized by gold and sulfur compounds and further by a mercury compound.
JP 04-009,034 discloses silver halide photographic element chemically sensitized by gold in the presence of an iridium or platinum complex salt.
JP 04-009,837 discloses silver halide photographic element chemically sensitized by reduction sensitization and further by a palladium compound.
EP 476,345 discloses silver halide photographic element chemically sensitized in the presence of a noble metal, a sulfur sensitizer, a selenium compound and a silver halide solvent.
JP 04-051,232 discloses silver halide photographic element chemically sensitized in the presence of a gold compound, an iridium compound, and a Group VIII metal compound.
JP 05-045,768 discloses silver halide photographic element chemically sensitized in the presence of a tellurium organic compound and a gold compound. JP 05-053,234 further discloses the possibility of an additional reduction sensitization.
JP 05-045,769 discloses silver halide photographic element chemically sensitized in the presence of tellurium, sulfur and noble metal organic compounds.
JP 04-335640, 05-027,360, 05-027,388, EP 563,708 and EP 638,840 disclose silver halide photographic element chemically sensitized in the presence of selenium, gold and sulfur sensitizer.
EP 568,092 discloses silver halide photographic element chemically sensitized in the presence of a heavy metal and a thiourea compound.
JP 06-051,418 discloses silver halide photographic element chemically sensitized in the presence of a mercuric chloride organic derivative, sodium thiocyanate, sodium thiosulfate, and potassium chloroaurate.
However, all sensitization methods that provide substantial increase of sensitivity are disadvantageous in that although high sensitivity is obtained, fog is also increased, particularly with tabular emulsions.
Additionally, photographic properties are generally altered when the material is subjected to different stressed conditions like rapid drying during manufacture, elevated temperature during storage, high temperature or roller pressure during processing in automatic machines.
After the sensitization process, the silver halide emulsion is coated on a support together with coating additives. A wide description of useful coating aids can be found in Research Disclosure No. 38597, September 1996, "Photographic Silver Halide Emulsions, Preparations, Addenda, Systems and Processing", Item IX.
U.S. Pat. No. 5,028,520 discloses the use of hydroquinone sulfonic acid potassium salt on tabular silver halide emulsion in an amount of from 0.03 to 0.5 moles per mole of silver to decrease the surface glossiness. The reference also states that no effect is obtained with an amount lower than 0.03 mole per mole of silver.
JP 54-040729, JP 56-001936 and JP 62-021143 disclose the use of polyhydroxybenzene derivatives on cubic silver halide emulsions to decrease pressure sensitivity in graphic art films.
EP 452772, EP 476521, EP 482599 and EP 488029 disclose the use of polyhydroxybenzene derivatives with functional groups that allow better silver halide grain adsorption to decrease pressure sensitivity of final film.
EP 339870 discloses a silver halide photographic emulsion having in reactive association a sensitizing amount of polyalkylene glycol compound and a fog reducing amount of an arylhydroxy compound.
The object of the invention is to prevent the above mentioned drawback and provide a silver halide emulsion with higher speed to Dmin ratio and enhanced film storability.